Recently, efforts have been made to increase the number of functions in a car and to make the car more luxurious. In order to implement the functions, an actuator such as a motor is provided at every portion of the car. Accordingly, a larger number of actuators are employed. Development of a small-size and low-cost actuator ECU (Electronic Control Unit) capable of controlling a large power to address such a situation is expected.
An H bridge circuit comprising four semiconductor switching-devices is employed in a number of cases as a circuit for driving the above actuator. A power transistor is typically employed as a semiconductor switching circuit. In general, a power transistor is required to have a noise-proof characteristic. In particular, since a power transistor mounted on a car is required to have a surge-proof characteristic, a power transistor having a high withstand voltage is employed. However, a power transistor having a high withstand voltage is built in a chip having a large size, which prevents miniaturization. In addition, since a power transistor with a high withstand voltage has a large on-resistance, heat is dissipated in a switching operation so that a countermeasure for coping with the dissipated heat must be devised. As a result, the use of such a power transistor is cumbersome.
In order to improve the surge-proof characteristic without employing a semiconductor switching-device having a high withstand voltage, the use of a circuit for carrying out protection in the event of a detected surge voltage is conceivable. For example, Japanese Patent Laid-open No. 11-341851 (referred to as patent reference 1), the contents of which are incorporated herein by reference, discloses a technology provided for a motor driven by an H bridge circuit. In accordance with this technology, the existence of an abnormality is confirmed when a voltage appearing at a power supply side terminal is not higher than a predetermined fraction of the voltage of a battery serving as the power supply or when a voltage appearing at a ground side terminal is not lower than another predetermined fraction of the voltage of the power supply and, in the event of such an abnormality, an operation to supply power from the battery to the motor is discontinued.
However, the technology disclosed in patent reference 1 has a configuration in which an abnormality of a motor is detected on the assumption that the voltage of the power supply is normal. Thus, the technology is not suitable for an application to protect semiconductor switching-devices employed in an H bridge circuit against a surge voltage caused by an excessive increase in power-supply voltage. For example, in accordance with the technology disclosed in patent reference 1, when existence of an abnormality in a motor is confirmed, a protection circuit cuts off the power supply path to the motor. However, the protection circuit requires a relay when semiconductor switching-devices are also to be protected against a large power caused by an applied surge voltage. Even if a relay is employed, however, the relay may not be capable of cutting off the path of a large current quickly with ease when the large current flows. In order to solve this problem, an expensive relay must be used. In addition, the use of a relay raises another problem of a configuration of large size.